Titanium alloy having high ductility, fatigue strength and rigidity and method of manufacturing same

ABSTRACT

A titanium alloy is provided wherein metal boride is uniformly crystallized and/or precipitated in the matrix. The heating temperature in the finishing hot working is set smaller than the β transus temperature by not less than 10° C., thereby causing the matrix to include an equiaxial α structure in a rate of not less than 40 vol %. This titanium alloy has excellent properties, i.e., high rigidity, ductility and fatigue strength, which are all required for structural components, and therefore can be widely applied to a mechanical component such as an engine of an automobile, a structural component in an aircraft as well as a component for a high speed rail vehicle.

TECHNICAL FIELD

[0001] The present invention relates to a titanium alloy having a highductility, fatigue strength and rigidity, which alloy is used in amechanical component requiring excellent mechanical properties and alight weight as well, for instance, a connecting rod, valve, camshaft,crankshaft and push rod in an engine of an automobile or a structuralcomponent in an aircraft, a high-speed rail vehicle or the like. Thepresent invention also relates to a method of manufacturing such atitanium alloy.

BACKGROUND ART

[0002] A titanium alloy has excellent properties for the corrosionresistance and the heat resistance, along with a high mechanicalstrength and a lightweight property, so that an application of the alloyto various mechanical components in an automobile, an aircraft and ahigh-speed rail vehicle is now widely extending. However, titanium alloyhas a relatively small Young's modulus, i.e., about half of that in ironor steel materials. Accordingly, buckling and bending must be taken intoaccount when the alloy is used in such a mechanical structure. Forinstance, when the titanium alloy is used to a mechanical componenthaving a long axial length, such as a camshaft, a connecting rod or thelike, the cross section of the component must be increased in a designwork in order to obtain a required mechanical strength. However, suchdesign work makes it impossible to effectively utilize the specificproperties of the titanium alloy, i.e., the lightweight and the highmechanical strength.

[0003] In view of these facts, several investigations have been made sofar to enhance the Young's modulus of the titanium alloy by providing acomposite material into which fibers or particles having a high Young'smodulus are dispersed in titanium. For instance, in Japanese PatentApplication Laid-open No. 5-5142, a method of producing a titanium-basedcomposite material has been proposed, in which a TiB solid solution isdispersed into the matrix of the titanium alloy in a predeterminedvolume percentage. In this specification, it has been demonstrated thatthe production method is capable of providing a high mechanicalstrength, a high rigidity, and a high wearing resistance over a widerange from room temperature to a high temperature.

[0004] However, the composite material has a less plastic workability inthe production method proposed therein, and therefore the application ofa melting/casting method or a powder metallurgy method is prerequisitefor this material, thereby making it impossible to employ the compositematerial to a large sized structural component. Moreover, the findingregarding the matrix structure in the composite material has not beendisclosed, and therefore it is not clear whether or not the ductilityand the fatigue strength required for such a structural element cansecurely be obtained with the method proposed therein.

[0005] Furthermore, in Japanese Patent Application Laid-open No.10-1760, a particle-strengthened type titanium-based composite materialhas been proposed, in which material the matrix is formed by a α−β typetitanium alloy including TiB or TiC particles, and the structure iscontrolled so as to obtain a needle-shaped α phase structure. In thecomposite material proposed therein, however, TiB or TiC particles areused as strengthened ceramic particles and therefore the powdermetallurgy method is prerequisite for the production method, therebymaking it difficult to apply the composite material to a large-scalestructural elements. In addition, the needle-shaped structure in thematrix provides a high Young's modulus. Nevertheless, a sufficientlyhigh ductility can hardly be obtained.

DISCLOSURE OF INVENTION

[0006] As described above, there is a problem that titanium alloy has arelatively higher mechanical strength, but a smaller Young's modulus,compared with the iron or steel materials. Various composite materialshave been produced to overcome this problem. However, no improvement hasbeen succeeded yet to obtain a high hot workability and a highductility.

[0007] On the other hand, it is required that the structural elementsmay be used in a much severer environment and the manufacturing cost mayalso be reduced, along with an excellent hot workability and mechanicalstrength. For instance, a high hot workability, a high rigidity, anexcellent ductility and fatigue strength are all required for aconnecting rod of an automobile, although it can be used in such a severenvironment and the manufacturing cost is further reduced. Nevertheless,any titanium alloy having such properties has not developed yet.

[0008] In view of these requirements on the development of titaniumalloys for such a mechanical part, it is an object of the presentinvention to provide titanium alloy having an excellent properties withregard to the hot workability, the ductility, the fatigue strength andthe rigidity, and it is further another object of the present inventionto provide a method of manufacturing such a titanium alloy. Morespecifically, an object of the invention is to develop a titanium alloywhich is capable of hot forging or hot rolling, and which has a tensilestrength not less than 1100 MPa and a Young's modulus not less than 130GPa, together with a provision of the ductility and fatigue strength ina predetermined magnitude.

[0009] The present inventors studied on the composition of elements, thefine particles to be dispersed and the structure in the matrix in orderto develop titanium alloys having the above-mentioned properties, andobtained the following findings (a) to (c):

[0010] (a) The Young's modulus of a titanium alloy may be effectivelyenhanced by dispersing particles having a high Young's modulus into amatrix. The dispersed particles are titanium carbide or titanium borideparticles, which are produced by the crystallization and/orprecipitation in the matrix. In this case, titanium boride is moreeffective in usage, since it has 1.3 times greater Young's modulus thantitanium carbide.

[0011] (b) In a titanium alloy, various matrix structures appear even ifit includes the same alloy composition. Fundamentally, these structurescan be classified into the equiaxial α structure and the needle-shaped αstructure. In order to obtain an excellent ductility and fatiguestrength, the matrix structure must have a certain rate of equiaxial αstructure.

[0012] In the formation of the equiaxial α structure in the matrix, itis necessary to carry out a thermal treatment after a working stress isapplied thereto. The temperature in the hot working should be smallerthan the β transus temperature. Moreover, it is preferable that thesubsequent solution treatment should also be carried out at atemperature smaller than the β transus temperature.

[0013] (c) Elements Al, oxygen (O), C, H and N, which serve to stabilizethe α phase, enhance the Young's modulus of the matrix, when they areincluded therein at an appropriate content. Moreover, neutral typeelements Sn, Zr and Hf provide a very weak effect on the enhancement ofthe Young's modulus, but an appreciate effect on the enhancement of themechanical strength at a high temperature and the creep resistance.

[0014] When an aging treatment is applied to the titanium alloyincluding the above-mentioned elements, Al, oxygen, or Sn, Zr, Hf, theseelements provide an aged hardening property of promoting to generate anintermetallic compound (Ti₃Al), thereby enabling the fatigue strength tobe greatly increased.

[0015] Complete solid solution or isomorphous type elements V and Moamong the β phase stabilizing elements greatly reduce the Young'smodulus, whereas eutectoid type elements Fe and Cr reduces not sogreatly, compared with the isomorphous type elements. At any rate, the βphase stabilizing elements reduce the Young's modulus to greater or lessextent, but enhance the hot workability. Accordingly, it is desirable toadd these elements to the alloy in an appropriate manner.

[0016] The present invention is realized on the basis of theabove-mention finding, and the gist is that the following titaniumalloys (1), (3) and (4), and the following methods of producing thetitanium alloys (2), (3) and (4) are provided:

[0017] (1) A titanium alloy having a high ductility, fatigue strengthand rigidity, wherein said titanium alloy includes B: 0.5-3.0% in mass%, and metal boride is uniformly crystallized and/or precipitated in thematrix, and wherein the matrix includes an equiaxial α structure in arate of not less than 40 vol %. The titanium alloy is either of α typeor of α+β type.

[0018] (2) A method for manufacturing a titanium alloy having a highductility, fatigue strength and rigidity, wherein the titanium alloyincludes B: 0.5-3.0% in mass %, and metal boride is uniformlycrystallized and/or precipitated in the matrix, and wherein the heatingtemperature in the finishing hot working should be set smaller than theβ transus temperature by not less than 10° C.

[0019] In the above manufacturing method, it is preferable that thesolution treatment should be applied within a temperature range between(the β transus temperature−350° C.) and (the β transus temperature−10°C.), and, if necessary, the aging treatment should be further applied.

[0020] (3) It is preferable that the above-mentioned titanium alloy (1)or (2) further includes Al: 5.5-10%, oxygen (O): 0.07-0.25%, C: not morethan 0.1%, H: not more than 0.05% and N: not more than 0.1% in weight %.

[0021] (4) Similarly, it is preferable that the above-mentioned titaniumalloy (3) further includes one or more than two of Sn, Zr and Hf in notmore than 20% in mass % in amount and/or one or more than two of β phasestabilizing elements in not more than 10% of V equivalent given by thebelow equation (a): $\begin{matrix}{{V\quad {equivalent}} = {V + {\frac{15}{10}{Mo}} + {\frac{15}{6.3}{Cr}} + {\frac{15}{4.0}{Fe}} + {\frac{15}{36}{Nb}} + {\frac{15}{9}{Ni}} + {\frac{15}{25}W}}} & (a)\end{matrix}$

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a table representing properties after various solidsolution treatments are applied to titanium alloys in Example 1; and

[0023]FIG. 2 is a table representing properties after various solidsolution or aging treatments are applied to titanium alloys in Example1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] A titanium alloy according to the invention is characterized byan excellent ductility and fatigue strength of the matrix structure, inwhich the rate of the equiaxial α structure (hereinafter denoted by “theisometric rate”) is controlled into an area rate (the same as the volumerate) more than 40% by finely and uniformly crystallizing and/orprecipitating metal boride in a matrix, and, if necessary, by includingone or more of the α phase stabilizing elements Al, oxygen and the likethereto.

[0025] Moreover, in the titanium alloy according to the invention, oneor more of Sn, Zr and HF is included therein to enhance the mechanicalstrength at high temperature and the creep resistance. Otherwise, theamount of β stabilizing elements to be added is restricted in anappropriate v equivalent so as not to form a β phase monolayer, and thusthe hot workability is enhanced by decreasing the β transus temperature.In the following, the reason for the above specification will bedescribed as for the microstructure, the element composition and themanufacturing method.

[0026] 1. Microstructure

[0027] Titanium alloy can be classified into three types in accordancewith the microstructure at normal temperature: α type; α+β type; and βtype. The subject matter of the present invention extends to the α typeand the α+β type.

[0028] Generally, either in the α type alloy or in the α+β type alloy,the equiaxial α structure is favorable for the ductility and the fatiguestrength, compared with the needle-shaped α structure. Furthermore, inaccordance with the author's investigation, it is found that the matrixof the alloy does not always need to be entirely constituted by theequiaxial α structure, and the mixture of the needle-shaped structuretransformed from the β phase therewith is allowed. However, in order toobtain a high ductility and fatigue strength in the mixed structure, itis necessary to set the rate of the equiaxial α structure, i.e., theequiaxial rate to be not less than 40% in the area rate. Furthermore, amore stable ductility and fatigue strength require an equiaxial rate ofnot less than 50% preferably.

[0029] The microstructure was inspected in the following steps: Aspecimen was collected from the matrix of the alloy and then observedafter polishing and etching. The area rate of the equiaxial α structure,i.e., the equiaxial rate which is defined in the present invention, isdetermined by the area ratio of the equiaxial α structure to theneedle-shaped structure, these structures being color-classified in theimage analysis of a micrograph of the matrix. The reason of theequiaxial rate used in the present invention is due to the fact that theductility and fatigue strength strongly depend on the area rate of theequiaxial α structure.

[0030] 2. Element Composition

[0031] B Composition:

[0032] In order to uniformly disperse metal boride (TiB) into the matrixof titanium alloy, B is added thereto and then crystallized and/orprecipitated in the course of solidification and cooling. Thereby, theYoung's modulus of the titanium alloy can be enhanced in accordance withthe composite rule in proportion to the magnitude of volume in TiBparticles having a greater Young's modulus than the titanium alloy.

[0033] A B content of less than 0.5% provides a reduced amount of TiBcrystallized and/or precipitated, thereby making it impossible tosufficiently enhance the Young's modulus of the titanium alloy. On theother hand, a B content of greater than 3.0% provides an excess amountof dispersed TiB and an enhanced Young's modulus of the matrix.Nevertheless, the hot ductility and the cold ductility are markedlyreduced. Accordingly, it is preferable that the content of B to be addedshould be 0.5-3.0%.

[0034] α Phase Stabilizing Elements:

[0035] Either Al or oxygen is a α phase stabilizing element, and has aprominent effect of solid solution hardening, thereby causing theYoung's modulus to be greatly enhanced. Either an Al content of lessthan 5.5% or oxygen content of less than 0.07% provides no suchsufficient effect. On the other hand, either an Al content of greaterthan 10% or an oxygen content of greater than 0.25% reduces theworkability and the ductility. As a result, it can be stated that thecontent of the two elements to be included should be set preferably, Al:5.5 to 10%; O: 0.07 to 0.25%, and more preferably Al: 7 to 9%; O: 0.07to 0.15%.

[0036] As another α phase stabilizing element, C, H or N can be used.All of these elements reduce the ductility at normal temperature.Therefore, the upper limit of the content should be set such that C:0.1%; H: 0.05% and N: 0.1%.

[0037] Neutral Type Elements

[0038] In the present invention, neutral type elements and/or β phasestabilizing elements may be added to the titanium alloy. In this case,any of these elements is solved in the matrix. Regarding neutral typeelements Zr and Hf, most amounts of these elements can be solved in thematrix, and a very small amount of zirconium boride and hafnium borideis crystallized and/or precipitated in the matrix. However, such a verysmall amount of the borides provides no prominent enhancement of theYoung's modulus.

[0039] One or more than two of the neutral type elements Sn, Zr and Hfcan be solved in the alloy. Sn, Zr or Hf provides no enhancement of theYoung's modulus, but enhances the effect of the solid solutionstrengthening to increase the mechanical strength at high temperature.More than 20% content of these elements reduces both the hot workabilityand the cold workability, and further increases the cost ofmanufacturing the alloy. Accordingly, the upper limit of the contentshould be 20% in amount, and preferably not more than 5%.

[0040] β Phase Stabilizing Elements

[0041] Elements V, Mo, Cr, Fe, Nb, Ni or W may be used as a β phasestabilizing element. The β phase stabilizing element included in thealloy decreases the β transus temperature and improves the hotworkability. These elements are solved in the matrix and suppress anexcessive generation of metallic compound (Ti₃Al), thereby enabling agreater content of Al to be solved. However, an excessive content ofthese elements causes the Young's modulus to be markedly reduced.Accordingly, one or more than two of these elements should be added tothe alloy within a range not more than 10% in the v equivalent given bythe below equation (a), and more preferably not more than 5% in the Vequivalent: $\begin{matrix}{{V\quad {equivalent}} = {V + {\frac{15}{10}{Mo}} + {\frac{15}{6.3}{Cr}} + {\frac{15}{4.0}{Fe}} + {\frac{15}{36}{Nb}} + {\frac{15}{9}{Ni}} + {\frac{15}{25}W}}} & (a)\end{matrix}$

[0042] 3. Manufacturing Process

[0043] The titanium alloy ingot is produced in the form of a compactshape of a raw material by appropriately selecting some of pure Al,electrolyzed Sn, Zr sponge, pure Hf, Al—V alloy, Al—Mo alloy and Mo, Cr,V and the like and by adding them to a titanium sponge in predeterminedcontents. In order to crystallize or precipitate TiB in the matrix ofthe titanium alloy in a dispersed state, Al boride, Fe boride or thelike is used as a boron source in the raw material. Moreover, the oxygenamount in the ingot can be adjusted to some extent by appropriatelyselecting the type of titanium sponge. When, however, a much greateramount of oxygen is required, TiO₂ can be used as an adjusting material.The raw material thus adjusted is arc-melted either by the consumableelectrode melting in a vacuum melting furnace or by the non-consumableelectrode melting in a plasma arc melting to form an alloy ingot.

[0044] The titanium alloy ingot thus produced is hot worked by forgingor rolling to obtain a desired microstructure, and then is appropriatelyheat-treated to adjust the mechanical properties. As described above, inorder to generate the equiaxial α structure in the matrix, the materialmust undergo a proper thermal history after applying a working stressthereto.

[0045] The structure in the matrix is widely changed by the heatingcondition at a temperature close to the β transus temperature. The hotworking at a temperature greater than the β transus temperaturefrequently generates the needle-shaped α structure, whereas the hotworking at a temperature smaller than the β transus temperaturefrequently generates the equiaxial α structure. Accordingly, in themanufacturing method according to the invention, the heating temperaturein the finishing hot working must be set smaller than the β transustemperature.

[0046] Since there exist the α and β phases in a mixed state within atemperature range just below the β transus temperature, the process ofcooling down to room temperature provides a mixed state of theneedle-shaped structure and the equiaxial structure. As described above,in order to obtain the ductility and fatigue strength in a predeterminedmagnitude by adjusting the equiaxial α structure at an area rate of notless than 40%, the heating temperature in the finishing hot working mustbe set smaller than the β transus temperature by not less than 10° C.There is no special limitation regarding the lower limit of the heatingtemperature. However, the temperature can be set greater than the lowerlimit temperature in the hot working. In the manufacturing methodaccording to the invention, the heating temperature in the finishing hotworking is specified such that a temperature greater than the β transustemperature can be used as for the heating temperature in the state ofthe rough work prior to the finishing work.

[0047] In other words, the hot working of the titanium alloy ingot isemployed not only to produce a predetermined profile of a structuralcomponent, but also to obtain a predetermined microstructure of thematrix. As described above, the heat treatment after undergoing theworking stress must be applied to generate the equiaxial α structure inthe matrix. Once, for example, the needle-shaped microstructure isformed, any heat treatment applied to the alloy no longer provides theequiaxial microstructure. In order to transform the needle structure ofthe matrix to the equiaxial structure, the hot working must again beapplied after the alloy is heated at a temperature smaller than the βtransus temperature.

[0048] In order to securely transform the needle structure of the matrixto the equiaxial structure, it is effective to provide a sufficientworking stress and it is referable that the hot working is carried outat a working rate not less than 50%. The crystallization and/orprecipitation of coarse TiB particles causes the ductility and thefatigue strength to be reduced. To avoid such reduction, it is necessaryto destroy the coarse particles by the hot working. In this case, theworking rate should be preferably not less than 70%.

[0049] In the titanium alloy, a decreased temperature for workingprovides a reduction in the hot workability as well as the generation ofworking fractures. To obtain a proper working temperature, either a heatinsulation material is coated onto the ingot, or the temperature in thecircumference is appropriately increased within a temperature range forthe warm working or the hot working, or the ingot is re-heated at atemperature smaller than the β transus temperature after the temperatureis decreased.

[0050] The titanium alloy thus hot worked undergoes such a heattreatment as a solution treatment and/or an aging treatment to adjustthe mechanical properties. When the temperature in the solutiontreatment is set smaller than the β transus temperature by not less than10° C., the equiaxial α structure, which is formed in the hot working,remains unchanged. On the other hand, a decreased temperature of thetreatment provides no effect of the solution treatment, so that thetemperature should be set not less than (the β transus temperature−350°C.). In accordance with the invention, the solution treatment should bemade preferably within a temperature range between (the β transustemperature−350° C.) and (the β transus temperature−10° C.), morepreferably within a temperature range between (the β transustemperature−200° C.) and (the β transus temperature−100° C.).

[0051] Moreover, the aging treatment promotes to generate theintermetallic compound (Ti₃Al), thereby enabling the fatigue strength ofthe titanium alloy to be further enhanced. The conditions of the agingtreatment vary from composition to composition of the alloy. It ispreferable that the temperature of treatment should be 500-600° C. andthe duration of treatment should be more than 5 hours.

EXAMPLES

[0052] The effect resulting from the invention will be described indetail, as for the case (Example 1), in which the solution treatment iscarried out after the hot forging, and the case (Example 2), in whichthe aging treatment is further applied to the above treatment.

Example 1

[0053] A titanium alloy having the composition shown in Table 1 wasarc-melted in a vacuum melting furnace to form an ingot having a 140 mmdiameter. The β transus temperature of the titanium alloy used in thetest was 1070° C. TABLE 1 Composition of elements (mass %) Al V Mo B O HTi 7.72 0.41 0.50 0.90 0.094 0.014 Bal.

[0054] By applying twice the hot forging and the solution treatment tothe alloy ingot obtained under the following conditions, test pieceswere produced:

[0055] 1. Rough-Forging

[0056] Size after forging: outside diameter 80 mm (working rate 68%,forging rate 3)

[0057] Heating temperature: 1170° C. (the β transus temperature+100° C.)

[0058] 2. Finish Forging

[0059] Size after forging: outside diameter 25 mm (working rate 90%,forging rate 10)

[0060] Heating temperature: 1040° C. to 1170° C. (the respective heatingtemperatures being indicated in FIG. 1)

[0061] 3. Solution Treatment

[0062] Heating temperature: 700° C. to 1100° C. (the respective heatingtemperatures being indicated in FIG. 1)

[0063] Heating duration: 2 hours

[0064] The tensile property at normal temperature, the fatigue strengthat normal temperature and the Yong's modulus were determined as theproperties of the titanium alloy used to test after the solutiontreatment. Furthermore, the microstructure of each test piece wasobserved to determine the isometric rate (vol. %) of the matrix. Theobtained results are given in FIG. 1.

[0065] From the results in FIG. 1, it is found that all the test pieceshave a tensile strength of 1100 MPa or more and a Young's modulus of 130Gpa or more, thereby exhibiting a high rigidity. In particular,inventive examples No. 3 to 6 provide an isometric rate of not less than40 vol % and further exhibit excellent properties regarding the fatiguestrength and the ductility, along with high rigidity.

[0066] In other words, a high ductility and high fatigue strength can beobtained without any reduction of high rigidity so long as the rate ofthe equiaxial α structure according to the invention is attained.

Example 2

[0067] Utilizing the alloy ingot obtained in Example 1, the effect ofthe aging treatment after the solution treatment was studied by varyingthe conditions of hot forging. The titanium alloys used to test weretreated according to the following processes A to D.

[0068] 1. Process A (Comparative Example)

[0069] 1-1. Finishing Forging

[0070] Size after forging: outside diameter 25 mm (working rate 97%,forging rate 30)

[0071] Heating temperature: 1170° C. (the β transus temperature+100° C.)

[0072] 1-2. Solution Treatment

[0073] Condition of treatment: 900° C.×2 hours

[0074] 2. Process B (Comparative Example)

[0075] 2-1. Finishing Forging

[0076] Size after forging: outside diameter 25 mm (working rate 97%,forging rate 30)

[0077] Heating temperature: 1170° C. (the β transus temperature+100° C.)

[0078] 2-2. Solution Treatment

[0079] Treatment condition: 900° C.×2 hours

[0080] 2-3. Aging Treatment

[0081] Treatment condition: 580° C.×8 hours

[0082] 3. Process C (Inventive Example)

[0083] 3-1. Rough-Forging

[0084] Size after forging: outside diameter 80 mm (working rate 68%,forging rate 3)

[0085] Heating temperature: 1170° C. (the β transus temperature+100° C.)

[0086] 3-2. Finishing Forging

[0087] Size after forging: outside diameter 25 mm (working rate 90%,forging rate 10)

[0088] Heating temperature: 1040° C. (the β transus temperature−30° C.)

[0089] 3-3. Solution Treatment

[0090] Treatment condition: 900° C.×2 hours

[0091] 4. Process D (Inventive Example)

[0092] 4-1. Rough-Forging

[0093] Size after forging: outside diameter 80 mm (working rate 68%,forging rate 3)

[0094] Heating temperature: 1170° C. (the β transus temperature+100° C.)

[0095] 4-2. Finishing Forging

[0096] Size after forging: outside diameter 25 mm (working rate 90%,forging rate 10)

[0097] Heating temperature: 1040° C. (the β transus temperature−30° C.)

[0098] 4-3. Solution Treatment

[0099] Treatment condition: 900° C.×2 hours

[0100] 4-4. Aging Treatment

[0101] Treatment condition: 580° C.×8 hours

[0102] The tensile property at normal temperature, the fatigue strengthat normal temperature, the Yong's modulus and further the equiaxial rate(vol %) of the matrix were determined as the properties of the titaniumalloy used to test after the solution treatment or the aging treatment.Furthermore, the microstructure of each test piece was observed todetermine the equiaxial rate (vol. %) of the matrix. The obtainedresults are given in FIG. 2.

[0103] In the processes A and B of the comparative examples, a tensilestrength of 1100 Mpa or more and a Young's modulus of 130 Gpa or morewere attained and a high rigidity was also obtained. However, animproper setting of the heating temperature in the finishing forgingprovided no sufficiently high ductility and fatigue strength. On thecontrary, in the processes C and D of the inventive examples, anexcellent ductility and fatigue strength were attained, along with ahigh rigidity. In the process D, moreover, an application of the agingtreatment enhances the proof stress and tensile stress and, at the sametime, greatly enhances the fatigue strength.

INDUSTRIAL APPLICABILITY

[0104] In accordance with the titanium alloy and the manufacturingmethod proposed in the present invention, excellent properties, i.e.,the rigidity, the ductility and the fatigue strength, which are allrequired for a structural component can be obtained, thereby making itpossible to provide mechanical components having excellent mechanicalproperties and a light weight as well. Accordingly, the titanium alloyaccording to the present invention can be widely applied to a mechanicalcomponent such as a connection rod, camshaft, crankshaft and push rod inan engine of an automobile as well as a structural element for anaircraft and parts for a high-speed rail vehicle.

1. A titanium alloy having a high ductility, fatigue strength andrigidity, wherein said titanium alloy includes B: 0.5-3.0% in mass %,and metal boride is uniformly crystallized and/or precipitated in thematrix, and wherein the matrix includes an equiaxial α structure in arate of not less than 40 vol %.
 2. A titanium alloy having a highductility, fatigue strength and rigidity according to claim 1, whereinsaid titanium alloy is either of α type or of α+β type.
 3. A titaniumalloy having a high ductility, fatigue strength and rigidity accordingto claim 1, wherein said titanium alloy further includes Al: 5.5-10%,oxygen (O): 0.07-0.25%, C: not more than 0.1%, H: not more than 0.05%and N: not more than 0.1% in mss %.
 4. A titanium alloy having a highductility, fatigue strength and rigidity according to claim 3, whereinsaid titanium alloy further includes one or more than two of Sn, Zr andHf in not more than 20% in mass % in amount and/or one or more than twoof β phase stabilizing elements in not more than 10% of V equivalentgiven by the below equation (a): $\begin{matrix}{{V\quad {equivalent}} = {V + {\frac{15}{10}{Mo}} + {\frac{15}{6.3}{Cr}} + {\frac{15}{4.0}{Fe}} + {\frac{15}{36}{Nb}} + {\frac{15}{9}{Ni}} + {\frac{15}{25}W}}} & (a)\end{matrix}$


5. A method for manufacturing a titanium alloy having a high ductility,fatigue strength and rigidity, wherein said titanium alloy includes B:0.5-3.0% in mass %, and metal boride is uniformly crystallized and/orprecipitated in the matrix, and wherein the heating temperature in thefinishing hot working is set smaller than the β transus temperature bynot less than 10° C.
 6. A method for manufacturing a titanium alloyhaving a high ductility, fatigue strength and rigidity according toclaim 5, wherein the solution treatment is carried out within atemperature range between (the β transus temperature−350° C.) and (the βtransus temperature−10° C.).
 7. A method for manufacturing a titaniumalloy having a high ductility, fatigue strength and rigidity accordingto claim 6, wherein the aging treatment is further carried out.
 8. Amethod for manufacturing titanium alloy having a high ductility, fatiguestrength and rigidity, wherein said titanium alloy includes B: 0.5-3.0%,Al: 5.5-10%, oxygen (O): 0.07-0.25%, C: not more than 0.1%, H: not morethan 0.05% and N: not more than 0.1% in mass %, and metal boride isuniformly crystallized and/or precipitated in the matrix, and whereinthe heating temperature in the finishing hot working is set smaller thanthe β transus temperature by not less than 10° C.
 9. A method formanufacturing a titanium alloy having a high ductility, fatigue strengthand rigidity according to claim 8, wherein the solution treatment iscarried out within a temperature range between (the β transustemperature−350° C.) and (the β transus temperature−10° C.).
 10. Amethod for manufacturing a titanium alloy having a high ductility,fatigue strength and rigidity according to claim 9, wherein the agingtreatment is further carried out.
 11. A method for manufacturing atitanium alloy having a high ductility, fatigue strength and rigidity,wherein said titanium alloy includes B: 0.5-3.0%, Al: 5.5-10%, oxygen(O): 0.07-0.25%, C: not more than 0.1%, H: not more than 0.05% and N:not more than 0.1% in mass %, and further includes one or more than twoof Sn, Zr and Hf in not more than 20% in mass % in amount and/or one ormore than two of β phase stabilizing elements in not more than 10% of Vequivalent given by the below equation (a), and wherein the heatingtemperature in the finishing hot working is set smaller than the βtransus temperature by not less than 10° C.: $\begin{matrix}{{V\quad {equivalent}} = {V + {\frac{15}{10}{Mo}} + {\frac{15}{6.3}{Cr}} + {\frac{15}{4.0}{Fe}} + {\frac{15}{36}{Nb}} + {\frac{15}{9}{Ni}} + {\frac{15}{25}W}}} & (a)\end{matrix}$


12. A method for manufacturing a titanium alloy having a high ductility,fatigue strength and rigidity according to claim 11, wherein thesolution treatment is carried out within a temperature range between(the β transus temperature−350° C.) and (the β transus temperature−10°C.).
 13. A method of manufacturing a titanium alloy having a highductility, fatigue strength and rigidity according to claim 12, whereinthe aging treatment is further carried out.